The present invention relates to a process for the preparation of isocyanates or isocyanate mixtures which are substantially free of color-imparting components and their use for the preparation of light-colored polyurethane rigid foams.
The effect of color on a polyurethane rigid foam is the basis of many experiments and papers. The lightest possible yellow to white shades are desired. In addition to a better visual impression, light-colored foams indicate that the raw materials from which they were produced have a high degree of purity. Undesirable streaks are formed on the surface of rigid foams if the starting materials are dark yellow or greyish. These streaks are produced by rising during the foaming process. The precise composition of the dyes or color-imparting components in polymers which cause discoloration of polyurethane foam has not yet been proven.
As the literature shows, elimination of the color problem is an essential part of isocyanate preparation. A number of processes to improve the color of polyurethane foams are described in the prior art. The use of dried magnesium silicates to remove traces of "color-imparting impurities" in diisocyanatodiphenylmethane (MDI) at about 190.degree. C. is disclosed in GB-A 2,207,671. In U.S. Pat. No. 4,465,639, the dark colored material produced on separation of the solvent during the preparation of diisocyanato-diphenylmethane is reduced by the. addition of small amounts of water. In U.S. Pat. No. 4,876,380, an extraction process in which the polyisocyanates are separated into a purified fraction with greatly improved color and a fraction where color is not a critical requirement is disclosed. This separation is achieved by using methylene dichloride and pentane. EP 0133538 also describes the purification of polyisocyanates by extraction. EP 0446781 A1 discloses a method for improving polyurethane foam in which diaminodiphenylmethane (MDA) is subjected to a catalytic hydrogenation in the presence of a hydrogenating catalyst and a particular amount of water. All catalysts known to be useful for catalytic hydrogenation are taught to be suitable as hydrogenating catalysts in this process. By-products are produced by this hydrogenation process. These by-products remain in the MDA and lead to color-imparting impurities in the MDI under unfavorable conditions for phosgenation. In addition, it is generally known that production of intensely colored compounds in the ppm range cannot be excluded due to the phosgenation reaction itself.